1. Field of the Invention
The present invention relates to an image signal processing device that processes a succession of images, an imaging device, and an image signal processing method.
2. Description of the Related Art
As to a succession of images, the amount of processing data is generally proportional to a frame rate.
Recording a succession of high-frame-rate images with a previous method requires a large amount of recording data. There is thus a demand for a recording method enabling a further reduction of an amount of recording data.
The recording method for capturing a succession of images is mainly classified into two types as follows:                record images as still images, and        record images as moving images.        
Considered first is a general recording method which records a succession of images as still images.
With such a method, the recorded still images are all completely separate, and they are not dependent on one another. Among the still images, there is usually no concept of the passage of time.
Accordingly, even with any images that are considerably similar to each other and showing little frame-to-frame motion, the images are separately recorded and stored each as a different still image. JPEG (Joint Photographic Experts Group) is exemplified for such a method.
Because the amount of recording data is proportional to a frame rate, this method causes increase of the amount of recording data if there is a high frame rate.
On the other hand, with a method of recording a succession of images as moving images, frames are continuously recorded at regular time intervals in accordance with a preset frame rate. During a sequence of moving images, no frame loss is allowed even if a single frame.
As such, the method requires frame recording with no frame loss irrespective of an increase or decrease of frame-to-frame motion.
That is, even with any images being considerably similar to each other and showing little frame-to-frame motion, for such a method using a fixed frame rate, there is no way of avoiding the increase of the amount of recording data if there is a high frame rate.
Motion JPEG or MPEG (Motion Picture Experts Group) Codec is exemplified for such a method.
If with Motion JPEG, because the frames are subjected only to intra-compression by JPEG, the amount of data is proportional to a frame rate, and the frame rate is fixed in value.
If with MPEG, e.g., MPEG2/4/H.264 AVC, the compression efficiency is increased due to compression using a frame-to-frame difference by inter-prediction, however, the frame rate is also fixed in value.